Frank Tepper, 45, had previously admitted to shooting his neighbor in Port Richmond, 21-year-old Bill Panas, during a fight in November, 2009.
However, Tepper's lawyer says the shooting was in self-defense.
In court on Tuesday, Panas' mother wept silently as the first witness described the chaos on Elkhart Street that night.
Police initially said Tepper was trying to break up the fight, but he was fired after an internal investigation.
Tepper, then a 16-year veteran of the force, was off-duty at the time of the shooting. He was dismissed from the department in January 2010 and charged a month later.
The trial got underway on Tuesday with opening statements complete by late morning.
Police say Tepper acted that night as a 'drunken participant' in a street fight who should have dialed 911. Tepper's lawyer asked the jury to consider whether Tepper fired in reasonable self-defense after getting pummeled by a drunken gang.
A friend of Panas, Chris Pichlo, testified that he and a group of neighborhood friends went to see what the commotion in front of Tepper's house was all about.
Within moments, the friend told the jury, Tepper - who was hosting a baby shower - emerged from a row home, gun in hand, and pointed it at him. Within seconds, Pichlo said, he saw Tepper shoot Panas in the chest.
"I pulled Bill off of some guy. He was just standing there and Tepper shot him from about three feet away," Pichlo said. When asked by the prosecutor if Panas was coming at Tepper, Pichlo said "No one was coming at Tepper at the time of the shooting. He appeared to be drunk, he was slurring his words."
On cross examination, Pichlo told the jury he and his neighborhood buddies had been smoking pot all day and had downed four or five beers before the deadly violence erupted just days before Thanksgiving.
